Does Tongwei photovoltaic panel need a battery

In cloud conditions, the panel output can be as low as 80%, meaning you will need a reliable battery system to hold energy stored for those days when the generation via solar isn't enough. [pdf]

How much tilt should photovoltaic panels have to be to be effective

To optimize performance, tilt your panels at an angle that is 10-15 degrees less than your latitude. For example, if you live at a latitude of 40 degrees, set your panels at about 25-30 degrees. [pdf]